Musculoskeletal tissues are composed of a composite of cellular and matrix components. In vivo, the cells are generally believed to be derived from undifferentiated cell lines that respond to different stimuli, both chemical and mechanical, and then ultimately differentiate and produce a particular matrix providing a tissue with a given structure and function. Furthermore, musculoskeletal tissues in living organisms have the ability to adapt to mechanical and physiologic changes throughout life.
An example is bone. The material properties of bone are governed by the density (and microdensity) of the material. The geometry of the bone determines its strength. The tubular structure of long bones provides them with a greater moment of inertia than would be true if bones were solid rods. Consequently, bones are stronger withstanding bending or torsional stresses than they would be if they were solid rods. As one grows older, the outer diameter increases as does the inner periosteal diameter. Theoretically, these changes allow one to maximize bone strength as bone mass decreases with age.
Articular cartilage is similarly composed of cellular and matrix components. The cells are uniquely isolated by the matrix and highly responsive to their environment. The matrix is composed primarily of collagen, proteoglycan, and water. The three-dimensional lattice and hydrostatic forces give cartilage its unique ability to withstand compressive forces.
In addition to what has been observed in-vivo, in-vitro studies have shown chondrocytes respond to mechanical loads (P. M. Freeman et al., J. Orth. Res., 12(3), 311-319 (1994)). This study found a decrease in the cell volume of chondrocytes in response to compressive loads. Other studies have shown an increase in proteoglycan synthesis and deposition in response to intermittent physiologic compression (G. P. J. van Kampen et al., Arthritis Rheum. 28 419-424 (1985)). Bone changes in response to load have been documented for many years and the appositional deposition of bone in an effort to increase the structural strength of loads areas is generally referred to as “Wolff's Law.” Similarly, tendon and ligament healing has been shown to be affected by the forces applied to these tissues at various periods in the healing process.
The last few years have seen a rapid increase in the number of biomaterials available to augment and enhance the bodies ability to repair and replace damaged musculoskeletal tissues. A recent article in the New England Journal of Medicine discusses autologous cartilage transplantation as a treatment of deep cartilage defects in the knee (M. Brittberg et al., New England J. Medicine, 331(14) 889-895 (1994). This method is currently available in the United States and undergoing investigation. The patient's cartilage is essentially “cloned” and reinserted in a cartilaginous defect after being grown in vitro to the appropriate volume. It is injected as a liquid paste and secured by an autologous periosteal patch. The authors had “encouraging” results in femoral condyle defects although the results were poor in the highly mechanically loaded patella.
Bone morphogenic protein, growth hormone, coral bone substitutes, bone paste, etc. are commercially available products used to enhance repair of fractures, nonunions, or osseous defects. These materials are gaining widespread acceptance within the medical community for their applicability in complex cases. Most of these products lack mechanical strength and structural properties approximate to the tissues they will support and rely on the healing of the host before adequate function can be restored
In U.S. Pat. No. 6,121,042, Peterson et al. disclose an apparatus for applying an axial load to a cultured tendon or ligament construct. However, the growth of bone is not disclosed. Further the application of torsional forces is not disclosed. Peterson further fails to disclose the application of forces scaled by (such as proportional to) a relevant elastic modulus of the cultured structure.